mpd-600-anp-10034-enu
TRANSCRIPT
© OMICRON Page 1 of 7
Application Note
Checking frequency properties of HFCTs
Author Dr.-Ing. Kay Rethmeier | [email protected] Date Sep, 2008
Related OMICRON Product MPD 600
Application Area Frequency check
Version v2.0
Document ID ANP_10034_ENU
© OMICRON 2013 Page 2 of 7
Content
1 Short Cut ................................................................................................................................................3
2 PD accessories have to fit to the PD system ......................................................................................3
3 Generate the frequency spectrum of your PD sensor .......................................................................3
4 Record the reference pulse ..................................................................................................................3
5 Record the pulse answer of the CT .....................................................................................................4
6 Import data to MS-Excel© ......................................................................................................................5
7 Plot frequency spectrum of the CT ......................................................................................................5
8 Frequency range OK .............................................................................................................................6
Please use this note only in combination with the related product manual which contains several important safety instructions. The user is responsible for every application that makes use of an OMICRON product. OMICRON electronics GmbH including all international branch offices is henceforth referred to as OMICRON. © OMICRON 2010. All rights reserved. This application note is a publication of OMICRON.
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The product information, specifications, and technical data embodied in this application note represent the technical status at the time of writing and are subject to change without prior notice.
We have done our best to ensure that the information given in this application note is useful, accurate and entirely reliable. However, OMICRON does not assume responsibility for any inaccuracies which may be present. OMICRON translates this application note from the source language English into a number of other languages. Any translation of this document is done for local requirements, and in the event of a dispute between the English and a non-English version, the English version of this note shall govern.
© OMICRON 2013 Page 3 of 7
1 Short Cut
1. Connect calibrator directly to the PD input of the MPD by short coax-cable
2. Generate file "CAL_absolute.TXT" from FFT
3. Connect HFCT to the PD input of the MPD
4. Short circuit calibrator THROUGH the CT
5. Generate file "CT_absolute.TXT" from FFT
6. Import TXT data files to MS-EXCEL©
7. Generate additional column "CT_relative" from "CT_absolute" minus "CAL_absolute"
8. Create diagram dB over frequency as a
2 PD accessories have to fit to the PD system
With the MPD 600 OMICRON developed a powerful PD Measuring System. One of its main benefits is its flexibility to use nearly any kind of PD sensor to decouple PD from the device under test (DUT). Thereby the sensor can be connected directly to the MPD PD input port without any additional electronic circuits needed. This will work properly if the output frequency range of the sensor can be covered by the input frequency range of the MPD. For sensors delivering output signals with significant frequency contend above 20 MHz an additional converter box is needed, as the MPD UHF Down-Converter for UHF PD measurements.
3 Generate the frequency spectrum of your PD sensor
If the data sheet of the HFCT does not contain a complete FFT curve of the sensors frequency range the OMICRON MPD 600 will be able to generate this curve. Therefore a reference pulse with a known frequency spectrum has to be injected to the PD sensor. The frequency output of the PD sensor, corrected by the characteristics of the input pulse, can be measured and displayed in the MPD FFT window.
4 Record the reference pulse
To record the reference pulse, simply connect the calibrator to the MPD PD input port by short coax-cable as shown in figure 1.
.
Figure 1: Connect Calibrator to MPD
© OMICRON 2013 Page 4 of 7
Get a stable pulse in the Time-window by selecting the trigger and display options in the PD-card. Change to the FFT-window and export the FFT data to a TXT-file named as "CAL_absolute.TXT".
Figure 2: Export FFT Data to TXT-File
This TXT-File will contain 1024 numeric values representing the FFT magnitude information scaled in dB from 0MHz up to 20 MHz.
5 Record the pulse answer of the CT
Now connect the output port of the CT to the PD input port. Take the calibrator and build a shortcircuit loop with a short measuring cable going through the window of the CT as shown in figure 3.
Figure 3: Inject Pulse into CT
This will inject a calibrator pulse into the CT, similar to a PD pulse to be measured with this sensor. If you decide to wind up more than one loop through the CT this has to be considered for the injected charge level. Select the Time-window and trigger on the received pulse to get a stable signal. Change to the FFT-window and export the measuring data (File "CT_absolute.TXT) as described above.
© OMICRON 2013 Page 5 of 7
6 Import data to MS-Excel©
To generate the resulting frequency spectrum curve of the CT both TXT-files created in the steps before have to be imported to a MS-EXCEL © data sheet. Therefore create 2 data sheets with the content of the 2 TXT-files and copy the data of one file to the other in order to get only one table with two columns. Add an additional column in COLUMN A with the 1024 missing frequency values 0 Hz to 20.000.000 Hz. Create a 4th column with the difference CT_ABSOLUTE minus CAL_ABSOLUTE, to be called CT_RELATIVE. This will give the corrected frequency spectrum of the CT, independent from the quality of the pulse source.
Figure 4: Export FFT Data to TXT-File
7 Plot frequency spectrum of the CT
Use the MS-EXCEL© diagram options to create a diagram according to your needs. Figure 5 shows an example scaled in dBm over logarithmic frequency range.
Figure 5: Frequency Spectrums of different HFCTs
Remember, the FFT always is a summary of the FFT-data of the CT, the calibrator and the MPD 600.
© OMICRON 2013 Page 6 of 7
8 Frequency range OK
As a High Quality Supplier of High-End PD Measuring Technique OMICRON suggests certain types of PD sensors to be operated with the MPD 600 Measuring System. If there is the need to use other types of sensors available on the market of PD measuring accessories, the MPD 600 will give the answer itself. The MPD FFT-window shows you the operating frequency range of the whole system as a superposition of the spectrums of the DUT, the PD sensor, the MPD and other signal propagation paths within the PD measuring circuit. If heavily attenuated areas in the FFT can be seen the PD measurement should not be performed in these specific filter windows. If there is no acceptable measuring window at all you may be forced to exchange the type of PD sensor to gain better results.
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